Veselago showed that incidence of light on a medium having a permittivity and a permeability both of negative values causes negative refraction and an artificial structure producing a negative permeability and a negative permittivity has been suggested. Such an artificial structure producing a negative permeability and a negative permittivity is an aggregate of structures having a scale sufficiently larger than atoms and smaller than a light wavelength and is called a metamaterial. Using the metamaterial as a negative refractive medium allows formation of a perfect lens having a planar shape. The perfect lens overcomes diffraction limitation to allow observation of a tiny object and allows accurate reproduction of a near field (evanescent wave).
A metamaterial is applicable to a lens for a terahertz electromagnetic wave having received attention in recent years. A terahertz electromagnetic wave is an electromagnetic wave having a frequency from 0.1 to 10 THz (wavelength from 30 to 3000 μm). This wavelength is substantially the same as a range from the wavelength of a far-infrared wave to that of a millimeter wave. The terahertz electromagnetic wave exists in a frequency range between the frequency of “light” and that of a “millimeter wave.” Thus, the terahertz electromagnetic wave has both an ability to identify an object with a spatial resolution as high as that of light and an ability comparable to that of a millimeter wave to pass through a substance. An electromagnetic wave in the terahertz wave band has not been explored so far. Meanwhile, application for example to characterization of a material has been examined that is to be achieved by time-domain spectroscopy, imaging, and tomography utilizing the characteristics of the electromagnetic wave in this frequency band. The terahertz electromagnetic wave has both the performance of passing through a substance and straightness. Thus, generating the terahertz electromagnetic wave instead of an X-ray allows safe and innovative imaging or ultrahigh-speed radio communication of some hundreds of Gbps.
In particular, terahertz imaging is one of quite attractive visualization techniques to take the place of an X ray for realizing safety, security, and high precision. Terahertz imaging has been reported to achieve terahertz nano-imaging in a near field overcoming diffraction limitation or reported to obtain a resolution of 400 nm (one wavelength divided by 540) at 1.4 THz. Terahertz imaging has also been reported to achieve imaging at 0.3 THz using a resonant tunneling diode. Using a metamaterial allows design of a negative refractive index n of −1 and is expected to achieve a flat perfect lens overcoming diffraction limitation by restoring near field light to become an evanescent component at a separate location.
A conventional sheet-type metamaterial 100 shown in FIG. 89 has been suggested (see non-patent literature 1). This sheet-type metamaterial 100 has a configuration like a flat plate formed by aligning a large number of unit cells 101 in a matrix periodically. As shown in a partial enlarged view of FIG. 89, the unit cell 101 includes a dielectric substrate 110 placed in an x-y plane. The dielectric substrate 110 has a front surface on which a front surface metal strip 111 of an elongated rectangular shape is formed to extend in an x-axis direction, and a back surface on which a back surface metal strip 112 of an elongated rectangular shape is formed so as to overlap the front surface metal strip 111. If a plane wave polarized in the x-axis direction enters the sheet-type metamaterial 100, flux linkage is generated between the front surface metal strip 111 and the back surface metal strip 112 formed on the opposite surfaces of the dielectric substrate 110. This causes a flow of a circulating current to make the front surface metal strip 111 and the back surface metal strip 112 function as a magnetic particle. In particular, an equivalent permeability takes a negative value at a resonant frequency of the front surface metal strip 111 and the back surface metal strip 112 or more. Further, polarization is generated by the application of an electric field E to make the front surface metal strip 111 and the back surface metal strip 112 function as a dielectric particle. In particular, resonance occurs between particles aligned in the x-axis direction at a given frequency and an equivalent permittivity takes a large positive value at a frequency not exceeding the given frequency. This generates a single-negative region between these resonant frequencies to attenuate the incident wave. A frequency rejection band in a given range can be obtained by selecting the dimensions or positions of the front surface metal strip 111 and the back surface metal strip 112 and adjusting the two resonant frequencies. For example, a frequency rejection band from about 4.5 to about 5.5 GHz can be obtained by setting dimensions as follows about the unit cell 101: a relative permittivity cr of the dielectric substrate 110 at 10.2, a breadth a, a height b, and a thickness c of the unit cell 101 at 15.2 mm, 12.7 mm, and 1.6 mm respectively, a length h and a width w of the front surface metal strip 111 and the back surface metal strip 112 at 12.1 mm and 0.6 mm respectively.